Using API to generate short link

I want to use a shortcode that generates a short URL from API of Premium URL Shortener script.

I added code from API to my template functions:

// Main Function
function shorten_url($url){

    $siteurl="http://go.mysite.com"; // FOR Example http://gempixel.com/short
    $apikey="8jkyA3e7xs1J"; // You can get it from the user account

    if($apikey && $siteurl){
        $short=@file_get_contents("$siteurl/api?api=$apikey&url=".strip_tags(trim($url)));
        $short=json_decode($short,TRUE);
        if(!$short["error"]){
            return $short["short"];
        }
    }
}
// Shortcode function
function shortcode_shorten_url($atts,$content){
   return shorten_url($content);
}

// Register Shortcode
add_shortcode("shorten", "shortcode_shorten_url");

Next I wanted to check if it works, so I added in template single code:

<?php echo do_shortcode('[shorten]http://mylink.com[/shorten]'); ?>

It works, I see a short link.

How can I use that short link in other places where I have external links, generated by template (functions.php), like below?

<?php  $numerado = 1; { while( have_rows('voo') ): the_row(); ?>
    <li class="elemento">
    <a href="https://wordpress.stackexchange.com/questions/217725/**I WANT TO PUT SHORT LINK HERE INSTEAD NORMAL LINK FROM op1**<?php echo get_sub_field("op1'); ?>" rel="nofollow" target="_blank" class="prawyklik">
    <span class="a"><b class="icon-controller-play play"></b> <?php _e('Option','mundothemes'); ?> <?php echo $numerado; ?></span>
    <span class="b">
    <img src="https://www.google.com/s2/favicons?domain=<?php echo get_sub_field('op2'); ?>" alt="<?php echo get_sub_field('op2'); ?>"> 
    <?php echo get_sub_field('op2'); ?>
    </span>
    <span class="c"><?php echo get_sub_field('op3'); ?></span>
    <span class="d"><?php echo get_sub_field('op4'); ?></span>
    </a>
    </li>
    <?php $numerado++; ?>
<?php endwhile; } ?>

You never need do_shortcode(), better said, almost never, there are a few cases where do_shortcode() could be appropiate.

Note how you could do just:

echo shorten_url( 'http://mylink.com ' );

instead of:

echo do_shortcode('[shorten]http://mylink.com[/shorten]');

Think about shortcodes as PHP functions placeholders; they are intended to be use where you can not execute PHP directly, like the content of post. But if you can execute PHP directly, like in a PHP script, there is no reason to use shortcodes, which is the placeholder, you can use the PHP function directly. There are a lot of benefits, meanly a better performance because do_shortcode() performs some preg regex that you don’t neet at all.

So, you can use the shortlink everywehere you need. For example:

<?php  $numerado = 1; { while( have_rows('voo') ): the_row(); ?>
    <li class="elemento">
    <a href="https://wordpress.stackexchange.com/questions/217725/<?php echo shorten_url( get_sub_field("op1') ); ?>" rel="nofollow" target="_blank" class="prawyklik">
    <span class="a"><b class="icon-controller-play play"></b> <?php _e('Option','mundothemes'); ?> <?php echo $numerado; ?></span>
    <span class="b">
    <img src="https://www.google.com/s2/favicons?domain=<?php echo get_sub_field('op2'); ?>" alt="<?php echo get_sub_field('op2'); ?>"> 
    <?php echo get_sub_field('op2'); ?>
    </span>
    <span class="c"><?php echo get_sub_field('op3'); ?></span>
    <span class="d"><?php echo get_sub_field('op4'); ?></span>
    </a>
    </li>
    <?php $numerado++; ?>
<?php endwhile; } ?>